Method of making a waterproof polyvinyl alcohol coated paper



Oct. 6, 1970 E. A. WOLF? 3,532,534

METHOD OF MAKING A WATERPROOF POLYVINYL ALCOHOL COATED PAPER Filed Feb. 9. 1967 AT TO-FZ- N EYS United States Patent US. Cl. 117-621 12 Claims ABSTRACT OF THE DISCLOSURE A process for treating a paper web having a coating which contains polyvinyl alcohol (PVA) adhesive to provide a waterproof surface without embrittlement of the web by contacting a dried PVA-coated web with an aqueous acidic solution.

BACKGROUND OF INVENTION Polyvinyl alcohol has recently experienced extensive use as a paper coating material. This has been due primarily to its easy solubility or dispersibility in water and its powerful adhesion to cellulose. Polyvinyl alcohol (PVA) is almost an ideal pigment coating adhesive, replacing heretofore widely used casein or starch. In comparison to these other adhesives, PVA not only has a higher binding power, but also provides coated paper stock with high brightness, good printability, light weight and other advantageous properties. However, PVA lacks an important property as a coating materialit does not pro duce a waterproof coating.

A number of proposals have been advanced to improve the waterproofness or wet-rub resistance of coatings containing PVA. Generally, these proposals involve curing PVA coatings with agents which either cure at high temperatures or require long curing periods, sometimes even followed by aging for days. Treatment of coated paper at excessively high temperatures, i.e., 225 F., or higher, dries out the paper to a bone dry condition, resulting in embrittlement which reduces the papers effectiveness in many applications. For example, the paper has poor folding properties and at todays high printing speeds, this embrittlement causes failure of the sheet in the printing press. When temperatures are lowered to avoid paper embrittlement, the curing cycle for making PVA coatings water resistant is extended and prohibits other finishing operations which normally would follow at high speeds.

Associated with the mentioned problems, prior art processes also experience difficulties caused by premature gelation of coating mixtures containing PVA and curing agents. Premature gelation severely increases the coating viscosity and makes it ditlicult to apply the coating to the substrate. Often the coated surface becomes mottled and contains imperfections. In short, prior art proposals for making a waterproof PVA-coated paper have proved to be unsatisfactory.

SUMMARY OF INVENTION The process of this invention for making a Waterproof PVA-coated paper includes the application of an aqueous PVA or PVA-pigment mixture to a paper web surface. The PVA-coated paper is first dried and then the coated surface is contacted with an aqueous acidic curing agent which insolubilizes the PVA to provide a waterproof surface without embrittlement of the web.

In a preferred form, the insolubilization is effected within a short time, i.e., on the order of seconds at room or relatively low temperatures, i.e., 100212 F. This low temperature treatment eliminates embrittlement of the paper web and its mentioned disadvantages. The aqueous 3,532,534 Patented Oct. 6, 1970 acidic curing agents employed in this process act quickly to insolubilize the PVA, i.e., on the order of several seconds up to 1 minute and even as fast as about 0.5 to about 1.5 seconds. This permits other paper finishing operations to immediately follow at high speeds. For example, the waterproof PVA-coated paper thus produced can be fed immediately and continuously to a supercalender operating at speeds as high as 600-1200 f.p.m.

After application of the insolubilizing solution to the PVA-coated web it is preferred to heat the web surface to temperatures of about 212 F. because excellent waterproofness can be advantageously effected at these temperatures and embrittlement is avoided. However, 212 F. does not set an exact upper temperature limit since temperatures above 212 F. have been found to permit rapid curing activity without embrittlement of the web.

The mentioned prior art problem of premature gelation of PVA coating mixtures is avoided by the process of this invention. The aqueous PVA or PVA-pigment mixtures are applied to a surface of the paper web without the active curing ingredients of this invention and then the coating is dried before cure is effected. Thus, there is no premature contact of PVA and curing agent which causes gelation of the coating mixture making it difficult to apply.

According to this invention, drying the paper Web after applying the aqueous PVA coating mixtures is necessary. The coated web is normally dried until it is dry to the touch. In this dried state, the coated web normally contains about 26% moisture. Although not completely understood, it is believed that the dried web does not permit deep penetration and surface dilution of the curing agents activity. Thus, the curing activity is concentrated at the surface of the Web to provide water-resistivity and excellent wet-rub resistance.

The extremely effective aqueous curing process of this invention has the further advantage of being readily integrated with the normal aqueous operations of commercial paper making. Therefore, employment of this process would not require extensive reconstruction or modification of paper mill machinery. Another important aspect of this invention is its provision of curing or insolubilizing systems which contain nontoxic and colorless curing agents. This avoids any chemical or physical masking of either the brightness of the PVA or colored pigments bound by the PVA.

Other and more specific features and advantages of the invention will appear from a detailed description, taken in connection with the accompanying drawing which illustrates by way of an example the process and one form of equipment which may be used in carrying it out.

The drawing is a schematic side view of a conventional supercalendar machine used for making coated paper according to this invention. Associated special equip- .ment is also shown for carrying the invention out in a preferred way.

Referring to the drawing in detail, the supercalender shown has multiple superposed rolls, indicated at 1-9 inclusive. Certain of the rolls have steel surfaces as indicated by the reference character S, and others have cotton or fiber surfaces of the conventional type as indicated by the reference character F. Fly rolls indicated at 10, 11 etc. guide the paper as it emerges from the nip between adjacent rolls into the next nip of rolls. A reel of coated paper containing the PVA adhesive or PVA-pigment coating is indicated at 20. The paper can be fed from its reel form 20 onto the top roll 1 of the calender and continued down through the calender onto a wind-up roll 21 driven in a conventional manner. The coated side of the paper contacts the steel calender rolls S. Between calender rolls 2 and 4 there is interposed an applicator roll and pan assembly having a revolvable roll-type coater 31 which is specially adapted to provide the insolubilizing or coating solution in accordance with this invention. The applicator assembly 30 is positioned before the cold fly roll and a heated fly roll 36 between calender rolls 2 and 4. The distance between calender rolls 2 and 4 through coater 30, fiy rolls 35, 36 is such that at paper coating speeds of about 400-1200 f.p.m. there is a sufficient lapse of time (about 0.5 to 1.5 seconds) to apply the curing agent to the coated paper and adequately insolubilize it before entering the next calender nip.

Example I Parts Clay 100 Polyvinyl alcohol (about 99.7 plus hydrolyzed) 3 Styrene-butadiene latex (Dow 3820 or 620 by Dow Chemical Co.) Urea-formaldehyde resin (Virset A 130 by Virginia Chemical and Smelting Co.) 0.75 Calcium stearate lubricant 1 Tetra sodium pyrophosphate 0.35

The coated sheet was suitably prepared on a paper coating machine in a conventional manner, then dried at about 2082l2 F. and reeled onto roll 20. The sheet was dry to the touch. The dried coated paper was then fed continuously into the calender over rollcoater 31 where the coated side was contacted with a 1% aqueous solution of aluminum chloride at conventional paper making speeds on the order of 650-750 f.p.m. After the coated paper was treated with the aqueous acid salt solution, it traveled to the cold fly roll 35 and then to the hot fly roll 36 heated at about l00212 F. About 0.5 to 1 second after the AlCl solution treatment, the paper entered the nip of the calender between rolls 3 and 4. There was no evidence of scumming or picking on the calender rolls which indicates by experience that the coating was adequately cured and dried before going to the calender nip. The web can then be optionally passed through the remaining nips of the supercalender to supply the finish desired for the product and then rewound onto roll 21. The water-resistant coating formed by the acidic salt solution treatment and heated fly roll is not altered by calendering. During calendering the web is not subjected to temperatures above the boiling point of water and preferably is kept in the range of about 100 to 212 F.

The paper taken off before or after calendering was not embrittled and had excellent folding properties. The coated sheet was tested for wet-rub resistance according to conventional test described below and responded with an excellent rating.

According to this test, a coated paper is placed on a sheet of black paper so that two or three inches of the black paper extend beyond the edge of the coated sheet. Next, a forefinger wet with water is rubbed 40 times over the coated surface (each of the 40 times constituting a stroke out and a stroke back or single strokes). Then, the forefinger is placed onto the black paper. This rubbing action serves to transfer varying amounts of the coating to the black paper depending upon the wetrub resistance of the coated sheet, leaving a white spot of certain intensity on the black paper when the coating dries. The intensity of this white spot will vary according to the water resistance of the coating. Ratings are conventionally given on a scale from excellent; very good; good; fair; poor to no detectable wet-rub resistance. Excellent denotes practically no visible white spot all) 4 on the black paper and represents excellent wet-rub resistance of the paper. Very good to no wet-rub resistance shows increasing whiteness from very slightly perceptible to exceedingly prominent, respectively.

Examples II-V A paper sheet was coated by hand with the following coating formula, on a parts by weight basis, with water added to make up a total solids content of about 55%.

Parts Clay Polyvinyl alcohol (98% hydrolyzed) 3 Styrene-butadiene latex (Dow 620 by Dow Chemical Co.) Urea-formaldehyde resin (Virset A by Virginia Chemicals and Smelting Co.) 1

Calcium stearate lubricant 1 Tetra sodium pyrophosphate 0.35

The sheet was dried for one minute at about 208- 2l2 F. Then, five separate samples of the coated sheet as prepared were treated with five different aqueous solutions of S Cl -5H O at 10%, 5%, 2.5%, 1.25% and 0.625% concentrations (percent by wt.). After drying at about 208212 F. for several seconds, the five samples of coated paper tested at excellent wet-rub resistance and were not embrittled.

The polyvinyl alcohol used in the process according to this invention can suitably be made from polyvinyl acetate by hydrolysis according to well-known techniques. A range of hydrolyzed polyvinyl alcohols can be used, for example, from the almost completely hydrolyzed product (less tran 1% acetate groups) to the partially hydrolyzed product (generally about 88% alcohol and 12% acetate).

The amounts of PVA employed in the coating formulations can be varied. When PVA and pigment are employed in the coatings, about 1 to 25 parts PVA can be used and it is preferred to employ about 1 to 8 parts PVA, both based upon 100 parts of pigment. Of course, it is within the skill of the paper maker to increase or vary the amount of PVA when used alone or with pigment over wide limits witrout critically affecting the operation of the process or departing from the spirit of this invention.

When preparing coating compositions, polyvinyl alcohol can be used in combination with other adhesives such as starch, casein or soy protein, and synthetic resins, the later usually in emulsion or latex form. Synthetic resin latices and emulsions can be used to impart desired properties to the paper web stock. These resins are generally characterized as thermoplastic in nature and tend to provide easier calendering when calender finishing is desired. They also may be used to impart higher smoothness, flexibility, dimensional stability and reduced curl. Examples of suitable synthetic resin adhesives include styrene-butadiene, acrylic resins, butadiene-acrylonitrile, polyvinyl acetate, copolymers of polyvinyl acetate, copolymers of polyvinyl acetate and acrylates, butadiene-methyl methacrylate copolymers and the like. Their selection, of course, for the mentioned purposes is within ordinary paper making skills. It is preferred to employ latices and emulsions from about 1 to 15 parts by weight based upon 100 parts pigment when they are blended with PVA coatings.

The aqueous insolubilizing agents of this invention are acid and acidic salt solutions. They can suitably be employed in concentrations of /2% to 10% over a pH range of about 0.5 to about 6.0. The strong acids having pHs of about 0.5 to about 3.0 will provide insolubilization but are very corrosive. The solutions of polyvalent metal salts of strong acids having pHs of about 3 to about 4.5 are preferred such as aluminum chloride, aluminum sulphate, chromic chloride, zirconium chloride, zinc nitrate, ferric chloride and stannic chloride.

Aluminum and zirconium acid salts capable of making substantially colorless or water White aqueous solutions are especially preferred for they provide that feature of this invention wherein insolubilization can be effective without chemically or physically masking the pigment or PVA to produce a nontoxic coated paper. Solutions of weaker acid salts at higher concentrations can be employed, such as ammonium sulphate in a pH range of about 4.5 to about 6.0. However, generally the weaker acid salts are comparatively more expensive than the salts of stronger acids and are less preferred.

Any of a number of well-known paper coating pigments can be employed with PVA adhesive or PVA adhesive blends as above mentioned. Clays of various types can be employed such as kaolin or china clay and other naturally occurring hydrous aluminum silicates. Other suitable pigments include calcium carbonate; titanium dioxide, satin white; phosphorescent pigments including zinc sulphide, cadmium sulphide and the like; and organic dyestuffs including the azo, azoic and vat dyes. Aqueous pigment dispersions or coating solutions can be facilitated by the use of dispersing agents. These dispersing agents aid in the preparation of the free flowing clay or pigment suspensions, particularly at high solids contents. Suitable dispersing agents are sodium silicate, sodium tetraphosphate, tetra sodium pyrophosphate, sodium hexametaphosphate and the like. The relative proportions of pigment to adhesive can be varied but, in general, the pigment comprises from about 75% to 97% (percent by wt.) based upon total pigment and adhesive.

Solids content of the PVA adhesive coating mixtures are quite flexible depending upon speeds required for paper manufacturing machinery. In general, solids content of the coating compositions used in accordance with this invention cover a range of about to 70%.

The aqueous PVA adhesive coating mixtures of this invention can be also supplemented with other curable ingredients. For example, the curable urea-formaldehyde and melamine-formaldehyde condensates of low molecular Weight can be employed. Even when these heat curable resin condensates are employed, coating cures can still be effected over a short period of time without higher temperatures or extended cure periods. When curing ingredients of the urea-formaldehyde type are included, they can be added in amounts of up to about /3 of the amount of PVA used.

The following examples serve to further illustrate the various aspects of this invention described above, but it is understood that these examples are not to be considered as limiting the scope and spirit of this invention.

Example VII A paper web stock was coated by hand with an aqueous coating suspension formulated on the identical basis as in example I, except that the polyvinyl alcohol employed was hydrolyzed to about 87-89%. After drying at about 208210-F. for 1 minute, the stock was treated on its coated side with a 1% solution of aluminum chloride and redried at about the same temperature for 1 minute. The coated paper tested at excellent wetrub resistance. This demonstrates that the process is applicable to partially hydrolyzed PVA as Well as highly hydrolyzed PVA.

Example VIII A paper wet stock was coated by hand with the aqueous coating suspension formula described in Example I and dried for about 1 minute at 208210 F. The dried coated web was then treated in two separate areas with aqueous nitric acid solutions of 1% to 2%. The stock was then dried at about 208-210 F. for several seconds. When subjected to the wet-rub resistance test, the coated sheet responded to an excellent rating in both areas treated with 1% and 2% nitric acid solutions.

6 Example IX A paper web stock was coated by hand with an aqueous coating composition prepared by mixing the following ingredients on a parts by weight basis with water to about 53% solids content.

Parts Clay Polyvinyl alcohol 6 Calcium stearate lubricant 1 Tetra sodium pyrophosphate 0.35

The coated web was dried to about a 2-6% moisture content. The web was then contacted with a 6% aqueous solution of aluminum chloride and subsequently dried at about 200 F. for about 0.5 to about 1.5 seconds. The resulting web tested at excellen wet-rub resistance.

Example X l A paper Web stock was coated by hand with an aqueous coating composition prepared on a parts by weight basis by mixing the following ingredients with water to about a 50% solids content.

Parts Clay 100 Polyvinyl alcohol 3 Styrene-butadiene latex (Dow 620 by Dow Chemical Co Calcium stearate 1 Tetra sodium pyrophosphate 0.35

The coated web was dried to about 26% moisture content and then contacted with a 1.5% acqueous solution of aluminum chloride. The web was then dried at about 200 'F. for several seconds. Its web-rub resistance was very good according to test.

What is claimed is:

1. A process for making a waterproof polyvinyl alcohol coated paper without embrittlement of the paper comprismg coating a surface of a paper web with an aqueous mixture consisting essentially of polyvinyl acohol, drying the coated web,

contacting the dried coated surface of the web with an aqueous acidic agent which insolubilizes the polyvinyl alcohol within a short time on the order of about 0.5 second up to one minute and drying or calendering the resultant web to provide a waterproof coating Without embrittlement of the web.

2. The process defined in claim 1 wherein said agent insolubilizes said polyvinyl alochol within said short time at a temperature in a range of about 100 F. to about 212 F.

3. The process defined in claim 2 wherein said time is in a range of about 0.5 to about 1.5 seconds.

4. The process defined in claim 1 wherein said agent comprises an aqueous solution of a polyvalent metal salt of a strong acid.

5. The process defined in claim 4 wherein said solution has a pH in a range of about 3 to about 4.5.

6. The process defined in claim 4 wherein said polyvalent metal salt is colorless and nontoxic.

7. The process defined in claim 6 wherein said salt is selected from the group consisting of aluminum chloride and zirconium chloride.

8. The process defined in claim 1 wherein said aqueous 7 8 acid in a concentration of /2% to 10% by weigh 12. The process defined in claim 9 wherein said resin said solution having a pH in a range about 3 to about is contained in an amount up to /3 parts by weight of the 4.5, which insolubilizes said polyvinyl alcohol in parts by weight polyvinyl alcohol. about 0.5 to about 1.5 seconds at a temperature in a range of about 100 F. to about 212 F. and drying or References Cited calendering the resultant web to provide a waterproof UNITED STATES PATENTS coating without embrittlement of the web. 10. The process defined in claim 9 wherein said aqueous 2691604 10/1954 Pnest 1l760 mixture also contains an adhesive selected from the group WILLIAM D MARTIN Primar Examiner consisting of casein, soy protein and synthetic resin latex m y and mixtures thereof. M. R. LUSIGNAN, Assistant Examiner 11. The process defined in claim 9 wherein said aqueous mixture also contains a curable formaldehyde resin selected from the group consisting of melamine-formaldehyde 1l765.2, 155 resin and urea-formaldehyde resin. 1 5 

